High-pressure gasification reactivity of biomass chars produced at different temperatures

The knowledge of biomass char gasification kinetics is of considerable importance in the design of advanced biomass gasifiers, some of which operate at high pressures. In the present work the effects of pyrolysis temperature, total pressure and CO₂ concentration on the gasification of biomass chars have been studied using the thermogravimetric approach. The chars were obtained by pyrolysis in a drop tube furnace reactor at temperatures of 1000 and 1400 °C. The gasification tests were carried out in a pressurized thermogravimetric analyser (PTGA) at different temperatures, pressures and CO₂ concentrations. The reactivity measurements were conducted under the kinetically controlled regime, and three nth-order kinetic models as well as the Langmuir–Hinshelwood model were applied to determine the kinetic parameters.     

Kinetics of Catalytic Hydrogenation of 5‐Hydroxymethylfurfural to 2,5‐bis‐Hydroxymethylfuran in Aqueous Solution over Ru/C

5‐Hydroxymethylfurfural (5‐HMF) is a cellulosic product of the hydrolysis of biomass, and it is widely considered for the production of several interesting chemicals and derivatives. In the present work, catalytic hydrogenation of 5‐hydroxymethylfurfural to 2,5‐bis‐hydroxymethylfuran was investigated using 5% Ru/C in the aqueous phase. Kinetic data were experimentally obtained over a wide range of temperatures (313–343 K), H₂ partial pressure (0.69–2.07 MPa), initial HMF concentration (19.8–59.5 mM), and catalyst loading (0.3–0.7 kg/m³) in a three‐phase slurry reactor. Disappearance of initial 5‐HMF concentrations was modeled using the power law and Langmuir–Hinshelwood–Hougen–Watson models. A model based on the competitive adsorption of molecular H₂ and HMF was proposed. It is presumed that surface reaction between nondissociatively chemisorbed H₂ and 5‐HMF was rate determining. This model provided the best fit for the kinetic data. From the Arrhenius equation, the activation energy for the surface reaction was found to be 104.9 kJ/mol.     

Isolation options for non-cellulosic heteropolysaccharides (HetPS)

The isolation of non-cellulosic heteropolysaccharides (HetPS) from barley husks (Hordeum spp.) and yellow poplar wood chips (Liriodendron tulipifera) was accomplished using mild steam explosion followed by extraction with water and ultrafiltration. The generally low yields, low purity, and low degree of polymerization (DP) improved when the HetPS were isolated following either alkali extraction of hammermilled or disk-refined biomass, or from holocellulose preparations generated by the conventional chlorite method or by organosolv delignification. Several purification methods were examined including precipitation using methanol; treatment with hydrogen peroxide (H₂O₂) or activated carbon (C) followed by precipitation with methanol; and H₂O₂-treatment followed by ultrafiltration. The isolation protocols were judged based on product yield, xylan content, and DP. The results indicate that, although steam explosion is effective in removing HetPS from the fiber source, virtually none remain in polymeric form. By contrast, alkali extraction succeeds in separating polymeric HetPS from the fiber source; and HetPS purity increases and polydispersity decreases with fiber prehydrolysis and delignification. Significant processing difficulties were attributed to the intimate association of HetPS with lignin which was effectively disrupted by acid-catalyzed pretreatment and treatment with H₂O₂. Ultrafiltration of H₂O₂-treated HetPS solutions represents the best procedure for isolating a xylan-rich polymer in high yield, with high DP and with high purity. Aqueous HetPS solutions can be spray- or freeze-dried into powderous products.     

Pyrolysis of a mixture of n-alkanes (chiefly C12-C18) atCA. 1053 K: Kinetic study

The thermal decomposition of a mixtures of n-alkanes, chiefly C₁₂-C₁₈ (commercial name Solpar) in the presence of steam (steam cracking) was studied in a plug flow reactor at ca. 1053 K. The tubular reactors were wound Incoloy steel tubes (I.D. 4 nm), heated by high-frequency induction. The total pressure in the reactor was slightly above atmospheric.The thermal decomposition mechanism of n-tetradecane, the major alkane in the Solpar mixture, allows the identification of all the primary products (non-zero initial rate of formation) by gas chromatography. Under these experimental conditions, very close to those of industrial stream cracking, secondary products (zero initial rate of formation) were observed and were qualitatively interpreted from elementary steps including the main primary products. This kinetic study also stresses the importance of the new steam cracking reactors calles “millisecond furnaces”, since these reactors produce high yields of light olefins and low yields of monoaromatic hydrocarbons.     

Sorption-enhanced steam methane reforming over Ni/Al2O3/KNaTiO3 bifunctional material

The performance of a new lab-made bifunctional material Ni/Al₂O₃/KNaTiO₃ for producing high purity H₂ via sorption-enhanced steam methane reforming (SESMR) was investigated. A series of bifunctional materials with 10 wt% Ni loading but different wt% ratios of KNaTiO₃ and Al₂O₃ was prepared by wetness impregnation method. All the materials were calcined at 700 °C for 3 hours and screened for their catalytic activity in a continuous flow fixed-bed reactor. The material containing 50 wt% each of KNaTiO₃ and Al₂O₃ (designated as HM) was found to be the best choice. The optimum process parameters for the production of high purity H₂ were determined: temperature = 700 °C, steam to carbon (S/C) molar ratio = 6 and gas-hourly space velocity (GHSV) = 2000 cm³ g^-1 h^-1. The values of CH₄ conversion, H₂ yield and H₂ purity were 87, 87 and 90%, respectively, at the optimum reaction conditions. The adsorption capacity of HM was found to be 14.7 wt%. With a breakthrough time of 10 min, the material was stable for 8 adsorption-desorption cycles. The regeneration of HM was achieved with N₂ gas at the same reaction temperature. Overall, the activity of this material for SESMR was very promising.     

Solar Hydrogen Generation from Lignocellulose

Photocatalytic reforming of lignocellulosic biomass is an emerging approach to produce renewable H₂. This process combines photo‐oxidation of aqueous biomass with photocatalytic hydrogen evolution at ambient temperature and pressure. Biomass conversion is less energy demanding than water splitting and generates high‐purity H₂ without O₂ production. Direct photoreforming of raw, unprocessed biomass has the potential to provide affordable and clean energy from locally sourced materials and waste.     

Novel Catalysts for Gasification of Biomass with High Conversion Efficiency

We have developed catalysts for the gasification of biomass with much higher energy efficiency than the conventional methods (noncatalyst, dolomite, and commercial steam reforming Ni catalyst). In our investigation, the Rh/CeO₂ gave high yields of syngas for cellulose gasification using a fluidized-bed reactor with batch feeding of cellulose. However, the catalyst was deactivated because of sintering of CeO₂ during the reaction. To prevent the CeO₂ sintering, we have further improved the catalyst and found that Rh/CeO₂/SiO₂ was quite stable for the gasification of cellulose. It was demonstrated that Rh/CeO₂/SiO₂ gave almost complete gasification of real biomass (wood powder) at 873 K. These results indicate that the combination of this kind of catalyst and fluidized-bed reactor can realize the energy-efficient biomass gasification.     

Elucidation of interaction among cellulose, lignin and xylan during tar and gas evolution in steam gasification

Time profiles of evolution rates of gas and tar in steam gasification of model biomass samples were examined using a continuous cross-flow moving bed type differential reactor to elucidate the interaction of the major biomass components (cellulose, xylan, lignin) during gas and tar evolution. Two types of model biomass samples (sample A: mixture of cellulose (65 wt%) and lignin (35 wt%); sample B: mixture of cellulose (50 wt%), xylan (23 wt%), and lignin (27 wt%)) were used for the experiment. In steam gasification of sample A, the evolutions of water-soluble tar and gaseous products (CO, H₂, CH₄ and C₂H₄) are significantly suppressed by the interaction between cellulose and lignin. The primary (initial) decomposition of lignin is hindered by the interaction with pyrolysate of cellulose. Then, the CO₂ evolution appreciably enhanced and the evolution of water-soluble tar delays. These results may imply that the volatilization of water-soluble tar derived from cellulose is suppressed by lignin and then the decomposition of char derived from polymerized saccharides and lignin takes place, emitting mainly CO₂. From the results using sample B, it was found that the addition of xylan greatly enhances the evolutions of gases (CO₂, CO, CH₄ and H₂) and accelerates the evolution of water-soluble tar and CO₂, implying that the enhancement of decomposition of water-soluble tar into gases and/or xylan decomposes into gases without significant interaction with cellulose or lignin. In addition, yields of the major tar components (levoglucosan, furfural and 5-methylfurfural) were measured using HPLC. It was observed that the interaction among cellulose, xylan and lignin suppresses the evolution of levoglucosan and significantly increases the evolution rate of 5-methylfurfural. There is an insignificant influence of interaction among cellulose, xylan and lignin for furfural evolution.     

Preliminary results on optimization of pilot scale pretreatment of wheat straw used in coproduction of bioethanol and electricity

The overall objective in this European Union-project is to develop cost and energy effective production systems for coproduction of bioethanol and electricity based on integrated biomass utilization. A pilot plan reactor for hydrothermal pretreatment (including weak acid hydrolysis, wet oxidation, and steam pretreatment) with a capacity of 100 kg/h was constructed and tested for pretreatment of wheat straw for ethanol production. Highest hemicellulose (C5 sugar) recovery and extraction of hemicellulose sugars was obtained at 190°C whereas highest C6 sugar yield was obtained at 200°C. Lowest toxicity of hydrolysates was observed at 190°C; however, addition of H₂O₂ improved the fermentability and sugar recoveries at the higher temperatures. The estimated total ethanol production was 223 kg/t straw assuming utilisation of both C6 and C5 during fermentation, and 0.5 g ethanol/g sugar.     

Kinetics of ethane oxidation

Ethane oxidation in jet-stirred reactor has recently been investigated at high temperature (800–1200 K) in the pressure range 1–10 atm and molecular species (H₂, CO, CO₂, CH₄, C₂H₂, C₂H₄, C₂H₆) concentration profiles were obtained by probe sampling and GC analysis. Ethane oxidation was modeled using a comprehensive kinetic reaction mechanism including the most recent findings concerning the kinetics of the reactions involved in the oxidation of C₁? C₄ hydrocarbons. The proposed mechanism is able to reproduce experimental data obtained in our high-pressure jet stirred reactor and ignition delay times measured in shock tube in the pressure range 1–13 atm, for temperatures extending from 800 to 2000 K and equivalence ratios of 0.1 to 2. It is also able to reproduce atoms concentrations (H,O) measured in shock tube at ≈2 atm. The same detailed kinetic mechanism can also be used to model the oxidation of methane, ethylene, propyne, and allene in similar conditions.     

Syngas Production from Propane Using Atmospheric Non-thermal Plasma

Propane steam reforming using a sliding discharge reactor was investigated under atmospheric pressure and low temperature (420 K). Non-thermal plasma steam reforming proceeded efficiently and hydrogen was formed as a main product (H₂ concentration up to 50%). By-products (C₂-hydrocarbons, methane, carbon dioxide) were measured with concentrations lower than 6%. The mean electrical power injected in the discharge is less than 2 kW. The process efficiency is described in terms of propane conversion rate, steam reforming and cracking selectivity, as well as by-products production. Chemical processes modelling based on classical thermodynamic equilibrium reactor is also proposed. Calculated data fit quiet well experimental results and indicate that the improvement of C₃H₈ conversion and then H₂ production can be achieved by increasing the gas fraction through the discharge. By improving the reactor design, the non-thermal plasma has a potential for being an effective way for supplying hydrogen or synthesis gas.     

Biomass pyrolysis: Kinetic modelling and experimental validation under high temperature and flash heating rate conditions

This work analyzes and discusses the general features of biomass pyrolysis, both on the basis of a new set of experiments and by using a detailed kinetic model of biomass devolatilization that includes also successive gas phase reactions of the released species and is therefore able to predict the main gases composition. Experiments are performed in a lab-scale Entrained Flow Reactor (EFR) to investigate biomass pyrolysis under high temperatures (1073–1273 K) and high heating fluxes (10–100 kW m⁻²). The influence of particle dimensions and temperature has been tested versus solid residence time in the reactor. The particle size appeared as the most crucial parameter. The pyrolysis of 0.4 mm particles is nearly finished under this range of temperatures after a reactor length of 0.3 m, with more than 75 wt% of gas release, whereas the conversion is still under evolution until the end of the reactor for larger particles up to 1.1 mm, due to internal heat transfer limitations. The preliminary comparisons between the model and the experimental data are encouraging and show the ability of this model to contribute to a better design and understanding of biomass pyrolysis process under severe conditions of temperature and heating fluxes typically found in industrial gasifiers.     

Methanation catalytic reactor

Storing renewable electricity in a natural gas grid is a new approach for seasonal storage. Using the existing natural gas infrastructure, a chemical energy source (methane) is stored efficiently, distributed and made available for use as required. Thus, the main step in the storage process is CO methanation. Modelling of an isothermal methanation catalytic reactor based on a kinetic scheme was carried out with Aspen plus™ software in a temperature range between 520 and 600 K and a H₂/CO molar ratio of 3, in the presence of CO₂ and steam. The model was validated by comparing simulation results with experimental ones. The maximum relative error is 10.87%. The effects of temperature, pressure and CO₂ addition in feed gas (syngas) on CO conversion and the outlet gas composition were carefully investigated.     

Reversible chemisorption of carbon dioxide: simultaneous production of fuel-cell grade H2 and compressed CO2 from synthesis gas

One vision of clean energy for the future is to produce hydrogen from coal in an ultra-clean plant. The conventional route consists of reacting the coal gasification product (after removal of trace impurities) with steam in a water gas shift (WGS) reactor to convert CO to CO₂ and H₂, followed by purification of the effluent gas in a pressure swing adsorption (PSA) unit to produce a high purity hydrogen product. PSA processes can also be designed to produce a CO₂ by-product at ambient pressure. This work proposes a novel concept called “Thermal Swing Sorption Enhanced Reaction (TSSER)” which simultaneously carries out the WGS reaction and the removal of CO₂ from the reaction zone by using a CO₂ chemisorbent in a single unit operation. The concept directly produces a fuel-cell grade H₂ and compressed CO₂ as a by-product gas. Removal of CO₂ from the reaction zone circumvents the equilibrium limitations of the reversible WGS reaction and enhances its forward rate of reaction. Recently measured sorption-desorption characteristics of two novel, reversible CO₂ chemisorbents (K₂CO₃ promoted hydrotalcite and Na₂O promoted alumina) are reviewed and the simulated performance of the proposed TSSER concept using the promoted hydrotalcite as the chemisorbent is reported.     

Experimental measurements and kinetic modeling of CH4/O2 and CH4/C2H6/O2 conversion at high pressure

A detailed chemical kinetic model for homogeneous combustion of the light hydrocarbon fuels CH₄ and C₂H₆ in the intermediate temperature range roughly 500–1100 K, and pressures up to 100 bar has been developed and validated experimentally. Rate constants have been obtained from critical evaluation of data for individual elementary reactions reported in the literature with particular emphasis on the conditions relevant to the present work. The experiments, involving CH₄/O₂ and CH₄/C₂H₆/O₂ mixtures diluted in N₂, have been carried out in a high‐pressure flow reactor at 600–900 K, 50–100 bar, and reaction stoichiometries ranging from very lean to fuel‐rich conditions. Model predictions are generally satisfactory. The governing reaction mechanisms are outlined based on calculations with the kinetic model. Finally, the mechanism was extended with a number of reactions important at high temperature and tested against data from shock tubes, laminar flames, and flow reactors. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 778–807, 2008     

Experimental measurements and kinetic modeling of CO/H2/O2/NOx conversion at high pressure

This paper presents results from lean CO/H₂/O₂/NO_x oxidation experiments conducted at 20–100 bar and 600–900 K. The experiments were carried out in a new high‐pressure laminar flow reactor designed to conduct well‐defined experimental investigations of homogeneous gas phase chemistry at pressures and temperatures up to 100 bar and 925 K. The results have been interpreted in terms of an updated detailed chemical kinetic model, designed to operate also at high pressures. The model, describing H₂/O₂, CO/CO₂, and NO_x chemistry, is developed from a critical review of data for individual elementary reactions, with supplementary rate constants determined from ab initio CBS‐QB3 calculations. New or updated rate constants are proposed for important reactions, including OH + HO₂ ? H₂O + O₂, CO + OH ? [HOCO] ? CO₂ + H, HOCO + OH ? CO + H₂O₂, NO₂ + H₂ ? HNO₂ + H, NO₂ + HO₂ ? HONO/HNO₂ + O₂, and HNO₂(+M) ? HONO(+M). Further validation of the model performance is obtained through comparisons with flow reactor experiments from the literature on the chemical systems H₂/O₂, H₂/O₂/NO₂, and CO/H₂O/O₂ at 780–1100 K and 1–10 bar. Moreover, introduction of the reaction CO + H₂O₂ → HOCO + OH into the model yields an improved prediction, but no final resolution, to the recently debated syngas ignition delay problem compared to previous kinetic models. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 454–480, 2008     

Surface Reaction Kinetics of the Oxidation and Reforming of CH4 over Rh/Al2O3 Catalysts

An 48‐step sur face reaction mechanism with thermodynamic consistent kinetic data is presented for the catalytic conversion of the gaseous chemical system H₂/O₂/H₂O/CO/CO₂/CH₄ over Rh/Al₂O₃ catalysts. Total and partial oxidation as well as steam reforming and dry reforming of methane over Rh catalysts is studied experimentally and numerically at varying temperature and composition. The results are used to extend the kinetic schemes we developed for H₂ oxidation, CO oxidation kinetics, and the water‐gas‐shift reactions in former studies. Aside from the experiments in a stagnation‐flow reactor presented here, we modeled a number of experiments from the literature to test the newly established kinetic scheme.     

一种适用于生物油低温制氢的碳纳米纤维促进的镍催化剂

氢气作为一种高热值的清洁能源广泛地应用于工业中. 研究证明: 生物质通过化学过程可以转化为多种气体燃料(氢气), 液体燃料以及高附加值的化学品. 生物质作为一种环境友好型再生洁净能源, 其研究越来越受到关注. 本文旨在探讨利用生物油为原料, 通过水蒸汽重整方法制备富氢合成气的过程. 利用均匀浸渍的方法制备了一种高分散的碳纳米纤维促进的镍(Ni/CNFs)催化剂, 并将普通的Al₂O₃作为载体的Ni/Al₂O₃催化剂和Ni/CNFs作对比. 研究了重整温度以及水蒸汽和碳摩尔比(n_S/n_C)对生物油水蒸汽重整制氢的影响. 结果表明: 碳纳米纤维作为载体用于生物油水蒸汽重整制氢的效果要远优于普通的Al₂O₃载体, 利用22% Ni/CNFs 催化剂时, 在实验温度范围内(350-550℃), 最高生物油转化率和氢气产率分别达到了94.7%和92.1%, 通过研究重整条件以及对催化剂进行表征探讨了生物油在水蒸汽重整过程中催化剂的构效关系.     

Removal of Nitrogen and Organic Matter in a Radial-Flow Aerobic-Anoxic Immobilized Biomass Reactor Used in the Posttreatment of Anaerobically Treated Effluent

This work reports on the removal of organic matter and nitrogen in a radial-flow aerobic-anoxic immobilized biomass (RAIB) reactor fed with domestic sewage pretreated in a horizontal-flow anaerobic immobilized biomass (HAIB) reactor. Polyurethane foam was used as support material for biomass attachment in both reactors. In batch experiments, a first-order kinetic model with residual concentration represented the organic matter removal rate, whereas nitrogen conversion followed a pseudo-first-order reaction in series model, with kinetic constants k ₁ (ammonium to nitrite) and k ₂ (nitrite to nitrate) of 0.25 and 6.62 h⁻¹, respectively. The RAIB reactor was operated in continuous-flow mode and changes in the airflow rate and hydraulic retention time were found to interfere in the apparent kinetic constants to the nitritation (k ₁) and nitratation (k ₂). Nitrification and denitrification were achieved in the partially aerated RAIB reactor operating with hydraulic retention times of 3.3 h and 2.7 h in the aerobic and anoxic zones, respectively. Ethanol was added in the anoxic zone of the reactor to promote denitrification. The effluent flow of the RAIB reactor presented a COD of 52 mg l⁻¹, and concentrations of 2 mg , 1.24 mg and 3.46 mg .     

Pyrolysis of Hailar lignite in an autogenerated steam agent

An in situ pyrolysis process of high moisture content lignite in an autogenerated steam agent was proposed. The aim is to utilize steam autogenerated from lignite moisture as a reactant to produce fuel gas and additional hydrogen. Thermogravimetric analysis revealed that mass loss and maximum mass loss rate increased with the rise of heating rates. The in situ pyrolysis process was performed in a screw kiln reactor to investigate the effects of moisture content and reactor temperature on product yields, gas compositions, and pyrolysis performance. The results demonstrated that inherent moisture in lignite had a significant influence on the product yield. The pyrolysis of L _R (raw lignite with a moisture content of 36.9 %, wet basis) at 900 °C exhibited higher dry yield of 33.67 mL g^?1 and H₂ content of 50.3 vol% than those from the pyrolysis of the predried lignite. It was also shown that increasing reaction temperature led to a rising dry gas yield and H₂ yield. The pyrolysis of L _R showed the maximum dry yield of 33.7 mL g^?1 and H₂ content of 53.2 vol% at 1,000 °C. The LHV of fuel gas ranged from 18.45 to 14.38 MJ Nm^?3 when the reactor temperature increased from 600 to 1,000 °C.